The present disclosure relates generally to power supply compensation, and more particularly to power supply parameter adjustment to improve light load operation.
DC/DC power converters can operate in a number of different modes, including continuous, discontinuous and transition modes. In continuous mode, also referred to sometimes as synchronous mode, an inductor that is charged and discharged has a substantially continuous current waveform, and may have negative current through the inductor. In transition mode, the inductor may have a continuous current waveform, and the inductor current is nonnegative. In discontinuous mode, sometimes referred to as nonsynchronous mode, the inductor current waveform is noncontinuous and the inductor current is nonnegative. In general, a DC/DC power converter can have improved efficiency at light loads when running in transition or discontinuous mode. In transition or discontinuous mode, negative inductor current can be prevented by using a low side power switch that turns off when inductor current reaches zero.
Referring to FIG. 1, a simplified circuit diagram of a power stage 100 of a conventional switching power supply is illustrated. Power stage 100 can be operated in discontinuous or transition conduction mode, in which current through an inductor 104 is prevented from becoming negative. A high side switch 101 charges inductor 104 when on. When a low side switch 102 is turned on, inductor 104 discharges, and current flowing through inductor 104 decreases toward zero. When the current through inductor 104 reaches zero, low side switch 102 is turned off, thereby preventing current in inductor 104 from becoming negative.
Synchronous or continuous conduction mode provides for inductor current becoming negative, so that inductor current is substantially continuous. Continuous conduction mode is typically used in heavy load operation to supply enough output current to meet the demands of the load. Illustrations of continuous mode waveforms and discontinuous mode waveforms are provided in FIGS. 2a, 2b, respectively.
A challenge in operating power stage 100 is to switch between continuous conduction mode and discontinuous conduction mode due to transitions between heavy and light loads on the output. For example, in discontinuous conduction mode, low side switch 102 is typically turned off before high side switch 101 turns on. In general, the situation in which high side switch 101 and a low side switch 102 are both on should be avoided to avoid cross-conduction problems, which may lead to incorrect operation of power stage 100 as well as damage or destruction of components in power stage 100. When power stage 100 operates in continuous conduction mode, cross-conduction is avoided by causing switch 102 to turn off before switch 101 is turned on. This type of dead time control (not shown) provides a slight delay between when switch 102 is turned off and when switch 101 is turned on to avoid cross conduction. Similarly, a dead time can be provided between when switch 101 turns off and when switch 102 turns on to avoid cross-conduction.
It can sometimes be challenging to coordinate the insertion of dead time in switching events when power stage 100 changes from discontinuous conduction mode to continuous conduction mode, or vice versa. When power stage 100 is driven with a PWM signal, a front end of a pulse for turning on switch 101 can become clipped because of the time delay associated with dead time for turning off low side switch 102 prior to permitting switch 101 to be turned on. In discontinuous conduction mode, because low side switch 102 is already off when a pulse is provided to turn on switch 101, a relatively longer pulse is applied to switch 101, since there is no clipping associated with dead time for turning off low side switch 102 prior to permitting switch 101 to be turned on.
A difficulty arises when power stage 100 operates in relatively light load conditions in which the mode may be continuous conduction or discontinuous conduction, depending upon the load. In continuous conduction mode, slightly less power is delivered to the load because of the clipped on time of high side switch 101. In addition, slightly greater power is delivered to the load when operating in discontinuous conduction mode because the pulse applied to switch 101 is slightly longer in comparison with equivalent continuous conduction mode. Because of the differences in on time for high side switch 101 in continuous conduction mode and discontinuous conduction mode, power stage 100 can be caused to oscillate between continuous conduction mode and discontinuous conduction mode. This oscillation can be problematic for efficiency, component protection and input boosting, for example.